The present invention relates generally to thread rolling machines. More specifically, the present invention relates to a load measuring system for a thread rolling machine that generates feedback signals for controlling the load developed by the thread rolling machine. An operating method for a thread rolling machine is also disclosed.
The present application is based on Japanese Patent Application No. 2002-085446, which application is incorporated, in its entirety, by reference.
Heretofore, thread rolling machines were employed to form threads on a workpiece. More specifically, a die having a configuration that is the inverse, i.e., mirror image, of the desired screw thread was pressed onto a workpiece while rotating the workpiece numerous times. The thread rolling machines were usually manually controlled. In other words, during the rolling operation, machining conditions such as frame pitch adjustment, load, and compression time were determined based on the experience of the operator. Therefore, the machining conditions required during thread rolling could not be quantified, making it difficult, if not impossible, to reduce the variability in the machining conditions and, thus, the resultant machined thread variations flowing from the experience and perceptual differences among operators. In short, manually controlled thread rolling machines lead to variations in product accuracy at the lot unit level.
Take, for example, the case in which a so-called circular die-type thread rolling machine is employed in producing a threaded workpiece. In such a thread rolling machine, a hydraulic cylinder is frequently employed to supply the die-driving force needed to feed the die that applies the necessary thread rolling load to the workpiece. In an effort to obtain information that would permit reduced variability of the threaded end product, these hydraulically powered thread rolling machines permitted the load at the time that the thread rolling was actually being performed to be determined by measuring the hydraulic pressure within the hydraulic cylinder and, then, multiplying the surface area of the hydraulic cylinder by the pressure measurement.
It will be noted that the load applied during the thread rolling processed obtained from the hydraulic cylinder driving the die is actually the force delivered at the output side of the thread rolling machine. This load value contains inherent errors due, for example, to flexure of the structural members of the thread rolling machine and/or the characteristics of the hydraulic circuit, e.g., pressure drops in the line supplying the pressure gauge. Consequently, the actual load applied during the thread rolling operation is different from the load determined using the hydraulic cylinder. Thus, hydraulic cylinder pressure data is not well suited for performing load control during the thread rolling operation, since the pressure data will not permit thread rolling machine control at the desired degree of precision.
It will be appreciated that the use of stress and strain gauges in connection with various rolling and/or milling machines is known. For example, U.S. Pat. No. 4,615,197 to Allebach discloses the use of strain gages in connection with a flat die thread rolling machine for the purpose of ascertaining the alignment of the fixed and movable dies relative to one another. More specifically, the reaction force exerted on the fixed die in a vertical direction during the thread rolling operation is measured; the load applied between the fixed and movable dies is not measured. In contrast, U.S. Pat. No. 4,487,044 to Fapiano and U.S. Pat. No. 6,116,073 to Kajiwara et al. each disclose the use of a load cell in a rolling mill employed in directly measuring the force applied in working a piece of bar stock. It will be noted that the load cell(s) must be sized the support the entire working load of the rolling mill, rather than measuring the stress or stain induced in, for example, a hydraulic piston shaft indicative of the working load. Thus, the size and cost of the measurement system cannot be optimized.
What is needed is a load measurement system that accurately determines the load applied to a workpiece disposed in a thread rolling machine. It would be desirable in the load measurement system could be integrated into the thread rolling machine to permit either automatic of manual control of the load developed by the thread rolling machine when the actual load deviates from a predetermined load. It would be particularly advantageous if the thread rolling machine generated a feedback signal that could be applied in multiple load control schemes.